Feedback amplifier



Dec. 22, 1942. N. J. OMAN FEEDBACK AMlLIFER Filed June 29, 19404 l'mventor l Nils el. @man Gttorncg il, and a capacitancefresistance filter nexus. The output plied to an Patented Dec. 22, 1942 Nils .1. Oman,

of Delaware Heddon Heights, N. J., assigner io Radio Corporation o f America, a corporation Application June 29,1940, serial No. 343,148

s claims. (ci. 11s-171) This invention relates to ampliilers and especlally to an amplifier and network in which undesired noise-producing components arereduced by feedback.

Numerous amplifier vnetworks including feedback connections for reducing hum or other undesired components are known to those skilled in the art. While such feedback connections may be used, the reduction `of hum or noise iS often accompanied by excessive regeneration and instability. It is 'an object of the present invention to provide means for reducing undesired components by feedback connections in which the regeneration and stability of an amplifier may be controlled. Another object is to provide means in anampliler for feeding back compotentiometer is connected through a phase shifter 35, a resistor 31, a capacitor 33,y and a resistor 4I to the upper terminal of the-filter I3.

The undesired hum components are derived by coupling a rectier 43, through an amplifier 45 (which may generally be omitted) to the load source 23. The rectifier Aoutput includes a capacitor 41 and a resistor 43 which are shunt-connents of a determinable and self-selecting fl'e- Y quency so that undesired components of the same frequency may be reduced by degenerative feedback. Another `object is to provide a means or a method for reducing the hum in a radio' transmitter by selecting regeneratively the hum components in a preamplifier and degeneratively feeding the hum component of the transmitter to the preamplifier.

The invention will be described by referring to the accompanying drawing in" which Figure 1 is a schematic diagram of onev embodiment of the invention and Figure 2 is a graph illustrating the operating characteristics of the invention.

Referring to Fig. 1, an audio or other signal source I is connected to the input of a preamplifier 3 which is included within the broken lines 5. The amplifier 3 may include a pair of thermionic tubes 1, 3. The input of the first 1 of these tubes is connected through an input transformer ground. The output of the ilrst tube is coupled Y by a resistor-capacitor network I3 tothe input' of the second tube 3. The output of the second I3 to tube includes a transformer I1 which applies the signal currents to the transmitter I3. The transmitter is energized by a power source 2i, which includes hum, noise, or other undesired compoof the transmitter I3 is apantenna 23 or other load. v

The preamplifier 3 includes a stabilizing network conslstins of a resistor 23 and a capacitor 21 which are serially connected between the output of the second tube 3 and the cathode 23 of the first tube 1. The c` thode 23 is at a higher potential than ground because it is connected to ground through a resistor 3i. A potentiometer across the primary of the output The movable contact of the p0- 33 is connected transformer l1.

nected to form a filter. A slidable contact on the resistor 43 is connected through a series resistor 5I to the junction of the resistor 31 and capacitor 33 in the regenerative feedback path from the output of the second tube 3 to the input of the rst tube. f

The mode of operation is as follows: The preamplifier 3 is stabilized by the stabilizing connection including the resistor-capacitor (2i-21)` combination. The regenerative feedback connection from the potentiometer 33 in the output circuitof the second tube 3 to the input ofthe first tube 1 includes the shunt filter i3 and series .dlter including the capacitor 39-resistor 4I combination. By `way of example, this lter system 23, 33, 4i is designed to pass currents including the undesired hum components. While' a resistorcapacitor filter is ordinarily not very selective, in the present arrangement the selectivity may be made very great because the regenerative feedback increases the selectivity. Thus the gain of the preamplifier isgreatly increased at the undesired frequency by means of the regenerative feedback which is so selected that its maximum effect occurs at the frequency of undesired components.

If the transmitter i3 includes thermionic tubes whose cathodes are heated by a low frequency (for example, sixty cycles per second) alternating current, undesired hum components of twice the applied frequency will appear in its output. Currents corresponding t3 these components may be derived by rectifying the transmitter carrier and by filtering `the radio frequency from the rectifier current. A portion of the filtered current is applied, in a degenerative phase to the feedback path 31, 33, etc. The degenerative feedback, operating at the' hum frequency, may be made large enough to reduce substantially the hum in the transmitter I3. It should be realized that reducing the hum by a degenerative connection is not equivalent to reduction by bucking. In the former connection, the hum may be made decreasingly small but never eliminated; in the latter connection, the hum ,may be eliminated or even over-neutralized by applying opposing currents greater in amplitude and opposite in phase to the original hum currents.

The characteristic of the system is illustrated by the graph of Figure 2. In the graph curve A represents the amplitude-frequency characteristic of the amplifier without the degenerative connection. The gain at 120 cycles is duetothe regeneration. The shape of the curve at this frequency is due to the combination of the regeneration and frequency selection of the filters.

means connected'between saidv output and input The broken line curve B represents the overall characteristic of the system when the degenerative feedback is operating. It will be noted that the response at 120 cycles has been reduced 20.8`

db. It should be understood that the response may be adjusted to reduce the hum by about 50 db. which appears to be an upper limit due to the difficulty in maintaining a stable amplier gain.

Thus, the illustrated form of the, invention involves a feedback amplifier, which includes a stabilizing path, a regenerative feedback path, l

and a degenerative path. The feedback path is made selective at the frequency of the undesired current components by Ameans of the' re'-`,` generative connection acting through the lter.

39, 4|, I3. The undesired components are applied degeneratively to the system to minimize the undesired hum. By way of example, a number of the constants for the reduction of undef sired currents of a frequency c-f 120 cycles per second haveA been indicated on the drawing. It should be understood that the invention is not limitedto the precise frequencies nor to these constants. Furthermore, thev methodmay be v practiced with one or more tubes in the preamplifier, with or without the load amplifier 45,

vand with or without the phase shifters.y

I claim as my invention:

1. An amplifier including an input circuit and an output circuit, means including a filter se, lectively responsive at a predetermined frequency connecting said input and said output, said input and output connectingmeans providing a regenerative path, additional means connecting said input and said output to stabilize said amplier, and means for applying degeneratively an undesired current component to said regenerative path so that said component is applied at said predetermined frequency to reduce said undesired component to a minimum in said output circuit and to obtain a uniform frequency response.

for stabilizing said amplifier'.

4. An amplifier according to claim 2 including a capacitor and a resistor serially connected between said output and said input for stabilizing sid amplifier. 5. Ahum-reducing amplifier including in com- Y bination a-source of current including undesired hum components, said amplifier having an input vv c :ircuityand I,an output circuit, means regeneratively connecting said output and input, said connecting means including a filter selectively responsive to feedback yat the hum frequency,l

means` forfderiving currents corresponding in Hfrequencyto said hum, land means `for applying saidde'ii'ved currents to said connectingmeans y in a degenerative phase. U v

J6. A"' hum reducingvkamplifler including in combination a rsource of current having undesired components of a predetermined frequency,

said,y amplifier including an'input circuit and an output .circuit, means regeneratively connecting said output to' said input, said connection including resistorfcapacitor filters responsive at said predetermined frequency, means for deriving I from said sourcecurre'nts corresponding Ito said undesired components,- means f or' applying said lderived currents to said 'connecting' means in a degenerative phase, and said amplifier.,

v7. 'I'he method of reducing undesired noise components in an amplifier system which includes amplifying currents covering a band of frequencies, increasing the "amplification in a means for stabilizing predetermined range ofufrequencieswithin said band, deriving from said system currents including the' undesired noise components," and applying saidderivedycurrents tothe input of said amplifier to offset said increasing amplification.

f8.' The `method offredii'cin'gV undesired noise components in an amplifiernetwork which in- 2. An amplifier including an input circuit and cludes amplifying 'currents covering' a band of frequencies, increasing the amplification of said currents by applying regeneratively currents including a predetermined `range of frequencies, deriving from said network currentsv including the undesired noise currents within said' range of, frequencies and applying said derived currentsto the input of ysaid network in a degenerative phase to reduce .the amplification within said range of frequencies.'r y

' NILSl J. OMAN. 

